This invention relates to magnetic bubble domain expander and detection circuits which employ an applied rotating magnetic field plus the effect of a gradient in the spacing between the bubble material and a magnetic film for the manipulation, expansion and detection of magnetic bubble domains.
Memory storage in conventional magnetic bubble devices is usually accomplished and measured by the presence or absence of bubble domains, propagated and manipulated on a chip by the use of an overlay, magnetically soft, (Permalloy) circuit in conjunction with an in-plane rotating magnetic field in the presence of a bias magnetic field.
In storage devices, the first circuit employed for bubble propagation through the circuit utilized the well-known T-I bar patterns and Y-bar propagate elements such as described in "Magnetic Bubble Technology: Integrated-Circuit Magnetics for Digital Storage and Processing," edited by Hsu Chang, IEEE Press, 1975, Library of Congress Catalog #73-87653, pages 20 and 24, beginning with the paragraph entitled, "Field Access Devices-Rotating Planar Field."
Recently, a new family of bubble propagation patterns, usually referred to as "gap tolerant" or sometimes called "half disc" or "wide-gap" patterns, were described in the Intermag-MMM conference, Pittsburgh, Pennsylvania, June, 1976 by employees of Bell Laboratories, Rockwell International and Texas Instruments.
However, in the prior mentioned T-I and Y bar patterns, the minimum feature size, that is, the smallest dimension in the pattern, which, in these patterns is the gap between the T and I or Y bars, is approximately one-third to one-fourth of the bubble diameter. Since the resolution of limit of photolithography used in the process of forming the overlays is somewhere between 1 to 1.5 microns, it is evident that the smallest bubble diameter that can be used with these patterns is approximately 4.0 microns.
The gap tolerant patterns had the main advantage in that their minimum feature size, and again in this case the gap between the elements, is approximately one-half of the diameter, which for a selected bubble size, for example, 4.0 microns, leads to less stringent photolithographic requirements. Conversely, if one wishes to maintain stringent photolithographic requirements, then the smaller bubble sizes could be tolerated.
One explanation for the greater tolerance in the gaps in the gap tolerant pattern is that when the bubbles are located at the ends of the elements, all ends have the same polarity, so the bubbles do not have to go through a magnetostatic energy barrier to bridge the gap.
Thus, propagation circuits for two micron bubble diameter devices using wide gap patterns can be processed by means of optical exposure with an improvement in memory storage density by a factor of around 3 over the conventional T-I or Y bar overlays mentioned above. Thus, it can be seen that, although the memory device had been improved by the gap tolerant patterns, it is the minimum feature size in magnetic bubble propagation circuitry, which thus far in the prior art is the gaps between elements, which limits the size of the domains and thus the storage density of the devices.
A similar problem of a feature size and the limitations of the photolithographic processes existed in the prior art expander-detection circuits. Such circuits are explained in the aforementioned IEEE book, supra, at page 37 under the paragraph entitled, "Magnetoresistive Detection," in which a bubble is expanded in a direction transverse to propagation and detected in a fishbone magnetoresistor and then alternatively shrunk and returned to the storage loop, or separated and portions thereof discarded. This latter eliminated the space for shrinking the bubbles and the return line to the storage loop. More specifically, the expansion or detection was accomplished on a number of V- or Chevron-shaped, closely but laterally spaced, elements in consecutive stages, increasing from a minimum number at an input stage, to a maximum number at a detection stage, then depending on the selected alternatives as mentioned previously, decreased successively to a minimum number so that the bubbles are first grown or elongated transverse to their direction of movement, then detected and then returned to their original size. Thus, a 6 micron domain at the input stage increases to about 1000 microns where the effect of the magnetic domain on the magnetoresistive material can be electronically detected.
However, as is apparent in connection with the memory propagate elements, as well as the detector-expander elements, it is the minimum feature size which limits the size of the domains and which limits the density of the elements in any given area. Accordingly, it is apparent that if the gaps between the elements of the expander-detector circuit can be eliminated, then the minimum feature size can be greatly increased. In such a case then, the feature size would not be the gaps but would be the width of the element itself-- approximately equal to two-thirds the bubble diameter. However, elimination of the gaps alone would not suffice since directionality of propagation through the expander-detector is imparted by means of the gaps which introduce asymmetry into the pattern. In other words, according to the prior art, no gaps, no directionality since it is the gaps that permit the formation of reversible poles in the propagate elements in response to the applied rotational magnetic field to attract the domains.
In the U.S. Pat. No. 3,927,398, issued to Magid Y. Dimyan on Dec. 16, 1975 and entitled, "Magnetic Bubble Propagation Circuit," it was shown that a translation force acting on magnetic bubble domains can also be produced by means of a gradient in the spacing between the bubble material and the propagate element. As described in this patent, an overlay of bars of uniform thickness of magnetic material were spaced from a film of bubble material with one end of the bar having a greater spacing than the other so as to form a gradient between the bubble material and the ends of each bar in the direction of propagation. With this pattern, a periodic monopolar magnetic field applied in the plane of the bubble material and parallel to the propagation path, magnetized the bars, causing the bubbles to move from one bar to an adjacent bar across the gap between the bars. When the bars were demagnetized, that is, the magnetic field ceased (being periodic), the bubble moved from the high end of the bar to the lower end. It is to be noted, however, that although the patented invention relied upon a spacing gradient to move the bubble from one end of the bar to the other, a gap between the bars was still required to obtain an overall directionality through the device. Thus, feature size was still the gap which limited the size of the domain and hence the storage density of the memory device, even though movement of the bubble domain was accomplished without an applied magnetic field at certain times, albeit only within the bars themselves. This patented invention did, however, eliminate the need of a rotating magnetic field used prior thereto in the prior art devices.